It has long been known that the reduction of iron ore to produce iron in a form suitable for steel-making can be carried out at temperatures below the melting point of iron by passing a hot reducing gas through a bed of particulate iron ore at temperatures of the order of 700.degree. C. to 1000.degree. C. to produce porous particles mainly composed of metallic iron. This is often referred to in the trade and technical literature as sponge iron or as direct reduced iron (DRI for short).
Because of its wide availability, natural gas has been extensively used as a source of the reducing gas for such iron sponge processes. This hydrocarbon gas is typically largely all methane. However, since methane per se is a relatively ineffective reducing agent for iron ore, it has been necessary to reform the methane by reaction with water or with carbon dioxide into a mixture of carbon monoxide and hydrogen for use as an ore-reducing gas. [See for example U.S. Pat. Nos. 3,765,872; 4,150,972, and 4,046,557, which all teach the use of a separate catalytic reformer external to the reduction reactor (some reformers being in, and others being out of, the reducing gas recycle loop)]
While reduction systems using reformed natural gas as a reducing agent have been extensively used commercially, they are open to the serious objection that the catalytic reformers they employ are costly pieces of equipment and form a substantial part of the investment and up-keep in such a sponge iron producing plant. Early attempts to eliminate external reformers were still tied to capital intensive accessory capital equipment; such as the coke ovens needed in U.S. Pat. No. 4,253,867, issued Mar. 3, 1981.
A process capable, or producing high quality sponge iron without using an external catalytic reformer or the like would substantially reduce the capital cost of such a plant. Such a system has been disclosed in the above-mentioned U.S. Pat. Nos. 4,668,284 and 4,528,030 (the subject matter of which is continued in this case) See also U.S. Pat. No. 4,880,458.
In the process disclosed in such patents, the natural gas is converted into hydrogen and carbon monoxide inside the reduction reactor using freshly reduced sponge iron as the catalyst. Water is injected into the reduction reactor (even though water is an oxidant) so as to promote the steam reformation of natural gas and to control carburization.
U.S. Pat. No. 4,432,789 granted to Mathisson et al., on Feb. 21, 1984, describes one method for minimizing some of the energy consumption in a reduction process to produce sponge iron. For this purpose, Mathisson proposes to recover the sensible heat (only) from the gas stream effluent from the reduction reactor to produce steam and utilize said steam for regenerating the washing liquid in the CO.sub.2 absorption system.
This concept had earlier been anticipated by Segre in U.S. Pat. No. 2,807,535 and by Koenig in German Laid-open application OS 1,911,052, published on Oct. 1, 1970.
However, the prior art fails to disclose any recuperation of the sensible and the latent heat present in the hot gas effluent from the reactor with such heat value being incorporated as hot water added back into the reducing gas recycled to the reactor. It also does not teach recovery of the hot water condensed from such hot gas effluent for such purpose. Even more specifically it does not collect and directly re-use the already hot and dirty quench water and condensed effluent water, supplying such hot water per se to humidify the gas stream fed to the reactor.